Is the correct number three, as stated in the Gemara in Sanhedrin, or four, as stated in the Midrash? The Chofetz Chaim suggests that neither is entirely accurate.

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All of Tefillah Being Specific in Prayer Birkas Kohanim During Shemoneh Esrei Changing a Decree After Rosh Hashanah Chazzan Who Mispronounces Words Conditional Prayers Davening for Israeli Soldiers Davening for Others With Feeling Davening for the Sick When Visiting Davening on a Raised Platform Davening Shemoneh Esrei Sitting Days Without Tachanun Forgetting HaMelech HaKadosh Forgetting Yaaleh VeYavo Four Considered Like the Dead Kavanah in Shemoneh Esrei Latest Time for Minchah Mi Shebeirach for an Unknown Status Modim DeRabbanan Prayer on Entering the Beis Midrash Praying About the Past Praying at Length Praying for Others With the Same Need Praying From Desperation Praying in Aramaic Alone Praying in English Reciting the Aseres HaDibros Daily Saying Korbanos Before Davening Selichos at Midnight or Early Morning Shabbos Shopping Before Davening Tefillah in Place of Korbanos Three Steps Back After Shemoneh Esrei Unanswered Prayers Unsure if You Said Vesein Tal Umatar Why Do We Pray